1. Field of the Invention
The invention relates to the control of internal combustion engines and, more particularly, relates to a method and apparatus for optimizing, by skip fire, the excess air ratio of a gas-fueled internal combustion engine.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, provide equal or greater power with equal or better mileage, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel as the primary fuel source in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn these fuels.
One drawback of gaseous fuels is that they have a relatively narrow range of useful excess air ratios or lambdas (defined as the ratio of total air available for combustion to that required to burn all of the fuel). In any fuel, if lambda drops below a minimum threshold, NO.sub.x and other emissions increase to unacceptable levels. On the other hand, if lambda rises above a maximum acceptable threshold, misfiring can occur, resulting in excessive unwanted emissions, and sharply decreased thermal efficiency. It is therefore essential for optimum control of combustion in an internal combustion engine to maintain lambda values within a permissible range, and preferably to cause lambda values to approach optimum levels.
Lambda control is particularly critical in gas fueled engines. Diesel engines have an extremely broad range of useful lambdas, ranging from about 1.3 at full power to about 10 at idle. Lambda control is rarely, if ever, required in such engines. On the other hand, referring to the curves 1.sub.1, 1.sub.2, and 1.sub.3 in FIG. 1 (illustrating lambda values at 1500 RPM, 1800 RPM, and 2100 RPM, respectively), in an engine using a small injection of pilot oil to ignite the pre-mixed air and gas by means of compression ignition of the pilot oil distributed throughout the gas/air mixture, the useful lambda range is much narrower (on the order of 1.2 to 3.0). This range decreases still further for a spark ignition engine--namely: lambda 1.2 to 1.6. The criticality of lambda control in gas engines is amply illustrated in FIG. 2, the curve 2.sub.1 of which illustrates that Nox emissions, as measured by brake specific NOx (BSNOx), are highest in both spark ignited and compression ignited gas engines at about lambda 1.1 and drop off dramatically at lambdas above about 1.3. Accordingly, the useful minimum lambda value is about 1.2. Thermal efficiency in spark ignited gas engines drops dramatically when lambda is greater than about 1.6 due to misfire (as represented by phantom curve 2.sub.3), requiring that lambda be maintained in a very narrow range for efficient engine operation. Curve 2.sub.2 illustrates that reducing lambda to maintain thermal efficiency is less critical for compression ignited gas engines, but is still important to prevent misfire as lambda approaches the maximum value of about 3.0 (not shown in FIG. 2).
Experiments on a Caterpillar 3406B dual fuel gas/diesel engine operated without skip fire showed the detrimental effects of low-load operation. Comparing the curve 4.sub.1 of FIG. 4 to curve 4.sub.2, the fuel efficiency of this dual fuel engine decreased by 10% or more at half load and high engine RPM when compared to the same engine operated at full load, while efficiency of a diesel-only engine remained essentially unchanged (see curves 4.sub.3 and 4.sub.4). Similarly, at low load, hydrocarbon emissions of a dual fuel engine were grossly unacceptable at 90 g/hp-hr or 5000 BTU/hp-hr (curve 5.sub.1 and CO emissions were relatively high at about 25 g/hp-hr (curve 5.sub.2). On the other hand, as illustrated by the curves 5.sub.3 and 5.sub.4, the corresponding levels for a diesel-only engine were relatively low.
While at least some of the potential beneficial effects of lambda control were known, an effective mechanism for controlling lambda was not. The air quantity and lambda of gaseous fueled engines utilizing pre-mixed air and gas were usually controlled by a throttle such as in typical gasoline fueled engines. A throttle, by its nature of causing a pressure loss in the intake system, is inefficient and greatly reduces engine thermal efficiency under part load and idle conditions. The idle fuel consumption of a throttled engine is about twice that of an unthrottled engine.
One technique for greatly reducing or eliminating throttle loss in an internal combustion engine is to selectively eliminate firing cycles (sometimes called "skip fire") by eliminating fuel supply and ignition from a selected number of the cylinders of the engine and to add the eliminated fuel to the remaining firing cylinders. This technique has been utilized in the prior art to improve engine performance, and is described extensively in the literature. However, all known prior art skip fire control schemes were configured without regard to lambda control.
For instance, starting as early as the turn of the century, skip fire has been used extensively for optimizing engine performance. In the early 1900's, nearly all stationary single cylinder Otto cycle engines were unthrottled and governed by skip fire. The intake mixture was pre-mixed fuel and air and maintained at an essentially constant lambda and inducted through an intake valve in the form of a spring-loaded check valve opened by the slight vacuum in the cylinder during the intake stroke. The engine power output and speed were controlled by a governor which held the exhaust valve open during overspeed conditions, thereby re-ingesting exhaust gas and consequently preventing the intake valve from opening and causing skip fire. At underspeed conditions, the exhaust valve was permitted to operate normally and caused the engine to injest a full charge of air fuel mixture on each cycle; hence, every firing was at full power and constant lambda until governed speed was again exceeded. It can thus be seen that skip fire control in these early systems was purely mechanical and was performed without regard to lambda for the sole purpose of controlling engine speed and power.
Skip fire is also disclosed in U.S. Pat. No. 2,771,867 to Peras, U.S. Pat. No. 4,504,488 to Forster, and Reissue U.S. Pat. No. 33,270 to Beck et al. While the control schemes of these patents, and particularly the Forster patent, is somewhat complicated, the criterion for the selection of the number of cylinders skipped is determined solely as a function of load, not lambda. Similar control schemes are disclosed in SAE Paper Nos. 930497 and 940548.
As another example, the 8,6,4 skip fire Cadillac engine, manufactured by General Motors in the late 1970s, used electro-mechanical deactivation of intake valves. The deactivation of cylinders in this system was also performed solely on the basis of load and had no effect on lambda. The Detroit Diesel 6V92 dual fuel gas/diesel engine also used skip fire, but the selection of number of cylinders skipped was rather arbitrary and not done as a specific means to optimize lambda. The fuel economy of this system was quite inferior to that of a base diesel engine as shown by a comparison of the curves 3.sub.1 and 3.sub.2 in FIG. 3. Power, on the other hand, was relatively unaffected by skip fire (See curve 3.sub.3).
The inventors of the present invention have also experimented with skip fire in the past, but not for lambda control. In addition to experimenting with the control scheme described in the Beck patent, they investigated the effect of skip fire on idle fuel consumption for a throttled, spark ignited GM 4.3 V-6 turbo lean engine fueled by natural gas. As illustrated by the bar graphs 6.sub.1 to 6.sub.6 in FIGS. 6a and 6b, these experiments revealed that, under no load conditions, THC (total hydrocarbon emissions) and fuel economy in a skip fire multipoint injection system improved significantly as compared to both multipoint and singlepoint injection systems without skip fire. Although skip fire was used in this system, the function was to maximize the efficiency by reducing intake manifold vacuum and increasing manifold pressure, not to control lambda per se. The selection of optimum fraction of cylinders firing (OFF) therefore was dependent on load demand. In addition, any lambda variation caused by skip fire in this system, as well as at least most of the remaining systems discussed above, was negligible since lambda was controlled by the throttle and gas fuel rate, not by skip fire.
The preceding discussion reveals that, although the techniques for achieving skip fire are contained in prior art, the strategy for how to select the optimum number of firing cylinders for lambda control is not. The inventors now realize that lambda can be optimized and engine performance improved dramatically by lambda control through skip fire.